Problem: In physics class, the girl to boy ratio is $4$ to $8$. If there are a total of $48$ students, how many girls are there?
A ratio of $4$ girls to $8$ boys means that a set of $12$ students will have $4$ girls and $8$ boys. A class of $48$ students has $4$ sets of $12$ students. Because we know that there are $4$ girls in each set of $12$ students, the class must have $4$ groups of $4$ girls each. There is a total of $16$ girls in physics class.